1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 5-163971 (JP-A-5-163971) describes a multi-cylinder internal combustion engine that includes a variable valve operating mechanism and a valve stop mechanism. The variable valve operating mechanism changes the valve characteristic of an engine valve, for example, an intake valve and/or an exhaust valve. The valve stop mechanism stops the opening and closing of the engine valve in at least one of cylinders.
In the internal combustion engine, an output from the engine is improved, and properties of exhaust gas are improved by variably controlling the valve characteristic of the engine valve using the variable valve operating mechanism. Also, for example, fuel efficiency is improved by performing a so-called reduced-cylinder operation. In the reduced-cylinder operation, the opening and closing of the engine valve (for example, the intake valve) in at least one of the cylinders is stopped using the valve stop mechanism, and thus, the at least one cylinder is deactivated.
In the case where a hydraulically-actuated variable valve operating mechanism is employed as the variable valve operating mechanism, a variable valve control that makes an actual value of a valve characteristic match a target value of the valve characteristic is executed by controlling a pressure of hydraulic fluid supplied to the variable valve operating mechanism. The variable valve control is executed taking into account cam torque acting on the variable valve operating mechanism due to a reaction force of a valve spring for the engine valve.
As described above, in the internal combustion engine in which at least one of the cylinders can be deactivated by stopping the opening and closing of the engine valve in the at least one cylinder, that is, in the internal combustion engine in which the so-called reduced-cylinder operation can be performed, when an all-cylinder operation is performed, all the engine valves are opened and closed, and thus all the valve springs generate the reaction force. On the other hand, when the reduced-cylinder operation is performed, the opening and closing of at least one engine valve are stopped, and thus, the reaction force generated by the valve springs is decreased according to the number of the engine valves whose opening and closing are stopped.
Thus, when the all-cylinder operation is performed, the reaction force generated by the valve springs is different from that when the reduced-cylinder operation is performed. Thus, when the all-cylinder operation is performed, for example, the average value of the cam torque acting on the variable valve operating mechanism is different from that when the reduced-cylinder operation is performed. Therefore, in the internal combustion engine in which the operation is changed between the all-cylinder operation and the reduced-cylinder operation, the valve characteristic, which is variably controlled, may become unstable.